While various types of super absorbent polymers have beer proposed to date and are known to the art from literature and patents, it is polyacrylic acid type super absorbent polymers that have been practically leading in the art. The term "polyacrylic acid type super absorbent polymer" as used herein denotes a polymer containing an acrylic acid monomer unit of at least 50 mol %, which is substantially water-insoluble but is highly swellable with water. Such polyacrylic acid type super absorbent polymers include polyacrylic acid crosslinked polymers or copolymers, starch-acrylonitrile grafted polymer hydrolyzates, starch-acrylic acid grafted crosslinked polymers, and vinyl acetate-acrylic ester copolymer saponified products. In these Polymers and copolymers 60 to 90 mol % of the carboxyl groups thereof generally have their hydrogen atom substituted with an alkali metal.
Super absorbent polymers are usually supplied for practical use in the form of powder or coating film. The performance of super absorbent polymers is evaluated in terms of absorptivity (absorption), rate of absorption, liquid Permeability, stability of swollen gel with time, and the like. Since many of these physical properties are conflicting with each other, it is very difficult to satisfy all these requirements, which has been a problem waiting for solution in further development of super absorbent polymers. For example, a super absorbent polymer having high absorptivity, i.e., a gel having a low degree of crosslinking, is inferior in rate of absorption, liquid permeability, and stability of swollen gel with time. On the other hand, a super absorbent polymer superior in rate of absorption, liquid permeability and stability of swollen gel with time, i.e., a gel having a high degree of crosslinking, tends to have low absorptivity.
In order to satisfy all the requirements, it has been proposed to improve practical performance by forming a highly crosslinked dense layer on the surface of a high absorbent polymer as disclosed in Japanese Patent Application Laid-opens 58-117222, 57-44627 and 63-99211, and Japanese Patent Publications 60-18690 and 61-48521. Japanese Patent Publication 5-19563, and Japanese Patent Application Laid-opens 61-211305, 61-264006 and 62-36411 disclose a method of graft treating a super absorbent polymer containing a carboxyl group and/or carboxylate group by using a silane coupling agent. Japanese Patent Application Laid-open 6-306118 proposes a method of treating with alkoxy titanium. Another known process comprises spraying an aqueous solution of a compound capable of readily reacting with the functional group, such as a carboxvlate group, of a super absorbent polymer, and then heating to form a highly crosslinked dense layer on a super absorbent polymer. Examples of the compound include a polyfunctional metal salt, a polyglycidyl ether and a polyisocyanate.
Another known process for producing a super absorbent polymer comprises adding to a super absorbent polymer a hydrophilic polymer having a reactive group and a crosslinking agent having at least two functional groups capable of reacting with the hydrophilic polymer and allowing the mixture to react by heating to coat the surface of the super absorbent polymer with the hydrophilic polymer with part of the functional group of the hydrophilic polymer being, crosslinked with the crosslinking agent. For example, Japanese Patent Application Laid-Open 60-36534 (corresponding to DE-A-3429379) discloses a process for producing a super absorbent polymer having a functional group on the surface thereof. According to this process, the aforesaid hydrophilic polymer is used in an amount as large as 1 to 30 parts by weight per 100 parts by weight of a super absorbent polymer to coat the surface of the super absorbent polymer, and a relatively small amount of a crosslinking agent is used to crosslink only a small portion of the hydrophilic polymer so that the functional group may remain effective. That is, the ratio of hydrophilic polymer to crosslinking agent is extremely high. For example, the ratio of the hydrophilic polymer to the crosslinking agent in the examples of JP-A 60-36534 is 40 to 180. This shows that the publication aims at employing a reactive group on the surface of a super absorbent polymer to change the quality of the super absorbent polymer, not at crosslinking the surface of the super absorbent polymer. Therefore, the process disclosed in the JP-A 60-36534 cannot provide a super absorbent polymer excellent in gel stability after swelling.
According to the process using a polyfunctional crosslinking agent capable of reacting with a carboxylate group, completion of the reaction requires a considerably long time, and strict conditions are imposed on adjustment of the water content of the super absorbent polymer at the time of crosslinking. In addition, it is necessary to moderately adjust not only the water content of the super absorbent polymer at the time of crosslinking but also the hydrophilic or hydrophobic properties and reactivity of the crosslinking agent in order to form a crosslinked layer in the vicinity of the surface layer of the super absorbent polymer, and it is extremely difficult to control the small thickness of the crosslinked layer. If the water content of the super absorbent polymer is reduced or if the crosslinking agent is rendered hydrophobic in order to form a thin crosslinked layer, the crosslinking efficiency of the crosslinking agent would be reduced or a desired crosslinking density could not be attained. Besides, a large proportion of the crosslinking agent, which is unfavorable for safety for use in sanitary materials, would remain in the final product.